A global demand for reduction of plastic waste has been increasing in recent years. The use of degradable plastics, especially biodegradable plastics is widely studied as a means to meet the above demand. Known examples of typical biodegradable plastics include polyester resins such as petroleum-derived polyglycolic acid, polylactic acid prepared using starch as a raw material, aliphatic polyesters prepared using biomass-derived diols and dicarboxylic acids as raw materials, and aliphatic polyesters produced by microorganisms, and aliphatic polycarbonates. These resins are microbially metabolized during composting thereof to be decomposed finally into carbon dioxide and water. In particular, biodegradable polyester resins are now relatively inexpensive owing to expansion of production and improvement of the production technology, and therefore are considered to be used in the place of a general-purpose resin that is hardly biodegradable and expected to be used for various applications. Such polyester resins however are problematically hard but brittle to have poor impact resistance, so as to be hardly used alone in the place of a general-purpose resin.
The flexibility, stretchability, and impact resistance are imparted to a hard polyester resin, for example, by adding a low-molecular-weight plasticizer as disclosed in Patent Literature 1, or by blending a soft aliphatic polyester such as polycaprolactone or polybutylene succinate with the hard polyester resin as disclosed in Patent Literatures 2 and 3. However, the low-molecular-weight plasticizer disclosed in Patent Literature 1 needs to be added in a large amount for exertion of a sufficient effect. In such a case, the plasticizer may bleed out to the surface with time to impair the appearance or lower the physical properties. The soft aliphatic polyesters disclosed in Patent Literatures 2 and 3 also need to be added in a large amount for achieving sufficient flexibility, resulting in reduction in other physical properties of the polyester resin (e.g., a significant decrease in yield stress or the like).